Organic Chemistry Fundamentals

Summary: Organic chemistry is the study of carbon compounds. Key concepts: homologous series, functional groups, isomerism, and systematic nomenclature. Compounds are classified by functional group. Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16

What is Organic Chemistry?

Organic chemistry is the study of compounds containing carbon. This excludes simple carbon compounds like carbon dioxide (CO2), carbon monoxide (CO), carbonates (e.g., CaCO3), and hydrogencarbonates — these are considered inorganic.

Why does carbon form so many compounds?

1. Each carbon atom forms 4 covalent bonds — it has 4 electrons in its outer shell (2.4), so it can form four single covalent bonds with other atoms
2. Catenation — carbon atoms can bond to other carbon atoms to form long chains and rings. This is the unique property that allows carbon to form the backbone of millions of organic molecules
3. Carbon can form single (C-C), double (C=C), and triple (C≡C) bonds

Hydrocarbons

A hydrocarbon is a compound containing only carbon and hydrogen atoms.

Examples: methane (CH4), ethene (C2H4), propane (C3H8) Non-examples: ethanol (C2H5OH — contains oxygen), chloroethane (C2H5Cl — contains chlorine)

Homologous Series

A homologous series is a family of organic compounds with:

• The same general formula (e.g., CnH2n+2 for alkanes)
• The same functional group (e.g., -OH for alcohols)
• Successive members differ by a -CH2- unit (relative molecular mass increases by 14 each step)
• Similar chemical properties (due to the same functional group)
• Trend in physical properties — as chain length increases:
  • Melting and boiling points increase (stronger intermolecular forces between larger molecules)
  • Viscosity (thickness) increases
  • Flammability decreases (longer chains burn less easily)
  • Volatility decreases (longer chains evaporate less readily)

Functional Groups Overview

A functional group is an atom or group of atoms that determines the chemical properties of an organic compound. All members of a homologous series share the same functional group.

Naming Organic Compounds (IUPAC Nomenclature)

Step 1: Identify the number of carbon atoms in the longest continuous chain — this gives the prefix:

Step 2: Identify the functional group — this gives the suffix:

Examples:

• CH4: meth- (1 carbon) + -ane (alkane) = methane
• C2H4: eth- (2 carbons) + -ene (alkene) = ethene
• C2H5OH: eth- (2 carbons) + -anol (alcohol) = ethanol
• CH3COOH: meth- (1 carbon in chain) + -anoic acid = methanoic acid (note: the -COOH carbon counts as carbon 1)

Types of Formula

Key distinction:

• The structural formula of butane is CH3CH2CH2CH3 — each carbon and the atoms attached to it are shown as a group
• The molecular formula is C4H10 — just the total count
• The empirical formula is C2H5 — the ratio 4:10 simplifies to 2:5

Isomerism

Definition: Isomers are compounds with the same molecular formula but different structural formulas (different arrangement of atoms).

Key facts about isomers:

• Same molecular formula → same Mr
• Different structural arrangements → different physical properties (different boiling points, melting points)
• Chemical properties may be similar or different depending on whether the functional group changes

Example 1: Isomers of C4H10

There are two isomers of butane (C4H10):

1. Butane (straight chain): CH3CH2CH2CH3 — boiling point: -0.5 °C
2. Methylpropane (branched): CH3CH(CH3)CH3 — boiling point: -11.7 °C

Both have molecular formula C4H10, but different structures → different boiling points because branched molecules have weaker intermolecular forces (less surface contact between molecules).

Example 2: Isomers of C5H12

There are three isomers of pentane (C5H12):

1. Pentane (straight chain): CH3CH2CH2CH2CH3
2. Methylbutane (one branch): CH3CH(CH3)CH2CH3
3. Dimethylpropane (two branches): CH3C(CH3)2CH3

When drawing isomers, start with the straight-chain version, then systematically move a -CH3 group to create branches. Make sure each isomer has the correct number of C and H atoms.

Fuels and Fractional Distillation

Petroleum (crude oil) is a mixture of hydrocarbons. It is separated into useful fractions by fractional distillation.

Process overview:

1. Petroleum is heated and vaporised
2. Vapour enters at the bottom of the fractionating column
3. Temperature is hottest at the bottom, coolest at the top
4. Different fractions condense at different heights depending on their boiling points
5. Long-chain hydrocarbons (high bp, low volatility) condense near the bottom; short-chain hydrocarbons (low bp, high volatility) condense near the top

Key Points

• Organic chemistry = chemistry of carbon compounds (excluding CO2, CO, carbonates, hydrogencarbonates)
• Hydrocarbon = compound containing only carbon and hydrogen
• Homologous series = family of compounds with same general formula, same functional group, successive members differ by -CH2-
• Functional group = atom/group of atoms responsible for characteristic reactions
• Isomers = same molecular formula, different structural formula → different physical properties
• Prefixes: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C)
• Suffixes: -ane (alkane), -ene (alkene), -anol (alcohol), -anoic acid (carboxylic acid)
• Fractional distillation separates crude oil based on boiling points; hottest at bottom, coolest at top of column
• C4H10 has 2 isomers (butane and methylpropane); C5H12 has 3 isomers

Key Concepts from Past Papers

• Fractional distillation depends on boiling point
• Petroleum is heated and vaporised; vapour enters at bottom of fractionating column
• Temperatures are high at bottom of column and low at the top
• Different fractions condense at different heights

Keywords from Past Papers

methane, ethene, structure, ene, point, bonds, carbon, properties, formula, fractions, energy, hydrocarbons, physical, boiling, idea

Related Notes

• Alkanes
• Alkenes
• Alcohols
• Carboxylic Acids
• Polymers

Sources

• OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Organic Chemistry], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry, Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 25: Organic Chemistry], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q44(c)(i) (2m), Q44(a)(i) (0m), Q44(a)(iii) (0m) (+2 more)
• 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
• 0620/32 Feb/March 2018: Q66(a)(i) (1m)
• 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
• 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
• 0620/32 Feb/March 2021: Q66(c)(i) (1m)
• 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
• 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
• 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
• 0620/33 May/June 2017: Q44(a)(ii) (1m)
• 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)

Common Misconceptions